In the autumn of 2014, nine large mesocosms were deployed in the oligotrophic subtropical North-Atlantic coastal waters off Gran Canaria (Spain). Their deployment was designed to address the acidification effects of CO2 levels from 400 to 1,400 μatm, on a plankton community experiencing upwelling of nutrient-rich deep water. Among other parameters, chlorophyll a (chl-a), potential respiration (PHi), and biomass in terms of particulate protein (B) were measured in the microplankton community (0.7–50.0 μm) during an oligotrophic phase (Phase I), a phytoplankton-bloom phase (Phase II), and a post-bloom phase (Phase III). Here, we explore the use of the PHi/chl-a ratio in monitoring shifts in the microplankton community composition and its metabolism. PHi/chl-a values below 2.5 μL O2/h/ (μg chl-a) indicated a community dominated by photoautotrophs. When PHi/chl-a ranged higher, between 2.5 and 7.0 μL O2/h/ (μg chl-a), it indicated a mixed community of phytoplankton, microzooplankton and heterotrophic prokaryotes. When PHi/chl-a rose above 7.0 μL O2/h/ (μg chl-a), it indicated a community where microzooplankton proliferated (>10.0 μL O2/h/ (μg chl-a)), because heterotrophic dinoflagellates bloomed. The first derivative of B, as a function of time (dB/dt), indicates the rate of protein build-up when positive and the rate of protein loss, when negative. It revealed that the maximum increase in particulate protein (biomass) occurred between 1 and 2 days before the chl-a peak. A day after this peak, the trough revealed the maximum net biomass loss. This analysis did not detect significant changes in particulate protein, neither in Phase I nor in Phase III. Integral analysis of PHi, chl-a and B, over the duration of each phase, for each mesocosm, reflected a positive relationship between PHi and pCO2 during Phase II [alpha = 23010−5 μL O2/h/L/(μatm CO2)/(phase-day), R2 = 0.30] and between chl-a and pCO2 during Phase III [alpha= 10010−5 μg chl-a/L/ (μ atmCO2)/ (phase-day), R2 = 0.84]. At the end of Phase II, a harmful algal species (HAS), Vicicitus globosus, bloomed in the high pCO2 mesocosms. In these mesocosms, microzooplankton did not proliferate, and chl-a retention time in the water column increased. In these V. globosus-disrupted communities, the PHi/chl-a ratio [4.1 +- 1.5 μL O2/h/(μg chl-a)] was more similar to the PHi/chl-a ratio in a mixed plankton community than to a photoautotroph-dominated one.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2020) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2020-11-30.